1. Field of the Invention
The present invention relates to a semiconductor device and a method of manufacturing the same, and more particularly, it relates to an insulated gate bipolar transistor (hereinafter referred to as IGBT) profitably used for an inverter and a method of manufacturing the same.
2. Description of the Background Art
In general, an IGBT device is formed by a number of parallel-connected IGBT elements (hereinafter referred to as IGBT cells). FIG. 22 is a sectional view showing a structure of a conventional n-channel IGBT cell.
Referring to FIG. 22, an n-type epitaxial layer 2 is provided on a p-type collector layer 1 consisting of a p-type semiconductor substrate. A p-type base region 3 is partially formed in the surface of the epitaxial layer 2 by selectively diffusing a p-type impurity, and n-type emitter region 4 is formed partially in the surface of the base region 3 by selectively diffusing an n-type impurity. A gate insulation film 5 is formed on the surface of the base region 3 between the surfaces of the epitaxial layer 2 and the emitter region 4. This gate insulation film 5 is provided so as to extend over the IGBT cells adjacent to each other. A gate electrode 6 of, for example, polysilicon, is formed on the gate insulation film 5, and an emitter electrode 7 of metal such as aluminum is formed to be electrically connected to both of the base region 3 and the emitter region 4. The gate electrode 6 and the emitter electrode 7 are commonly electrically connected to each cell forming the IGBT device. A collector electrode 8 of metal is formed on the back surface of the collector layer 1 in common with each of the IGBT cells.
The IGBT device is a voltage controlled transistor having an insulation gate (MOS gate) as well as MOS FET, and therefore it has the advantage of symplifying the structure of a drive circuit. The IGBT device comprises the collector layer 1 consisting of a p-type region for injecting a hole into the epitaxial layer 2, and hence a conductivity modulation effect arises in the epitaxial layer 2 by an injection of a hole from the collector layer 1, so that lower on-state resistance can be implemented in contrast to a MOS FET having high breakdown voltage. Because of above both advantages, the IGBT device is noted as the most desirable element for an inverter.
FIG. 23 shows an example of a three-phase inverter circuit having IGBT devices. As shown in FIG. 23, six IGBT devices 10 and a motor 11 as a load are connected between a positive feed terminal 8 and a negative feed terminal 9 to form a three-phase inverter bridge, and fly-wheel diodes 12 are connected in parallel to the IGBT devices 10 for improving the switching characteristic of the IGBT devices 10, respectively. The fly-wheel diodes 12 are generally fixed to the outside of a chip in which the IGBT devices 10 are formed.
In a case where each IGBT device 10 of the inverter circuit shown by FIG. 23 is constituted by the IGBT device as shown in FIG. 22, a carrier is accumulated in the epitaxial layer 2 when each IGBT cell is turned off. Since the life time of the carrier thus accumulated is long, the switching velocity of each IGBT device reduces.
In order to accelerate the switching velocity of an IGBT device, such an IGBT device, for example, as shown in FIG. 24 may be proposed.
As shown in FIG. 24, in this IGBT device, a p-type collector region 21 is formed by selectively introducing a p-type impurity into a first major surface of an n-type semiconductor substrate 20 through a method such as diffusion and the like, and an n.sup.+ -type region 22 is formed by introducing an n-type impurity into a region except for the collector region 21 within the first major surface of the semiconductor substrate 20. The same reference numerals as in FIG. 22 designate like parts, and hence further explanation thereof will be omitted.
In this IGBT device, an IGBT cell 23 is formed, within the section corresponding to the collector region 21, by the collector region 21, the semiconductor substrate 20, the base region 3, the emitter region 4, the gate insulation film 5 and the gate electrode 6. According to the IGBT device, when each IGBT cell is turned off, the carrier (hole) accumulated in the semiconductor substrate 20 is extracted to the collector electrode 8 through the n.sup.+ -type region 22 having a low resistance value, and therefore the switching velocity of the IGBT cell can be accelerated without controlling the carrier life time of the IGBT cell. However, owing to existence of the n.sup.+ -type region 22 a parasitic diode 24 is unexpectedly formed, within the section corresponding to the n.sup.+ -type region 22, by the base region 3, the semiconductor substrate 20 and the n.sup.+ -type region 22. Since the parasitic diode 24 has a long recovery time, it causes the switching velocity of the IGBT device to reduce, and therefore it is impossible to use the IGBT device of FIG. 24 as a high speed device.